The present invention relates oil well pump timer controls and more particularly to an oil well pump control which maintains a substantially constant on-off duty cycle when intermittent power outages prevent pump operation.
Prior art known to the applicant and believed to be relevant to the present invention includes U.S. Pat. No. 2,596,330 issued to Everard on May 13, 1952 and U.S. Pat. No. 3,953,777 issued to McKee on Apr. 27, 1976. The Everard patent discloses an electromechanical clock mechanism for duty cycle control of an oil well pump. The McKee patent on the other hand discloses current sensing apparatus for shutting off an oil well pump when a pumped off condition occurs as indicated by reduced motor currents.
Most oil wells require artificial lift equipment for the production of oil. Most artificial lift equipment comprises some type of electrically powered pump for lifting the oil from the bottom of the well bore to the surface tanks. The quantity of production from a well is determined almost totally by the natural conditions of flow of oil from the formation into the well bore. Ideally a well pump is sized to pump the oil from the well bore at the natural flow rate. But, since every well is somewhat different and every well's production changes with time, it has not proven practical to provide a pump having the exact production capability as the well. It is not desireable to have an undersized pump since this allows a head of oil to build up in the well bore and reduces the production rate. On the other hand an oversized pump will eventually pump the well bore dry and continued operation of the pump will damage the pump itself.
Various types of pump off controls have been provided for shutting down an oil well pump to prevent damage when the bore has been pumped dry. Some of these control devices detect fluid flow while others detect the motor current as taught in the above referenced McKee patent. The simplest and oldest form of well pump controller is the on-off duty cycle controller such as the timed switch taught by the above referenced Everard patent.
The on-off timer is still the most universally accepted well pump controller for several reasons. Control by time periods is easy to understand and the controllers are usually relatively inexpensive and easy to install and operate. Operators therefore can appreciate the commercial advantages of such devices and once having installed a time controller are unlikely to discard it in favor of a more expensive type of control system. But in practice several problems have arisen with the use of such on-off timers. Some of the timers are actually more complicated than required by allowing the setting of nonuniform on periods and nonuniform off periods. Maximum efficiency is actually acheived by providing on periods of sufficient length to pump the fluid down to pump inlet level and off periods of sufficient length to allow the well bore to refill. Such on and off periods may vary slowly but are essentially constant on a day-to-day basis. As a result the controller having a single presettable on time and single presettable off time is actually preferable to a controller allowing nonuniform times to be set during a twenty-four hour or seven day period.
A second and more serious problem occurs when a power outage shuts down the well during time periods when the well should be pumping. Stripper wells are often located in remote areas and are not provided with a reliable source of electrical power. An operator can determine in various ways that power has been turned off to his well he visits the well for inspection or removal of stored oil. Most electromechanical timing devices include a time indicator and when the indicated time is incorrect the operator knows that the power has been off. In response to such a condition operators often manually overrid the timing device and turn the well on continuously for a longer period of time than is desireable. For example the operator may turn the pump on and leave it on continuously until the next inspection trip. Such an operation almost insures a well will pump dry and some damage will be done to the pump. It can be seen that it would be desirable to turn the pump on continuously after a power outage for only a sufficient time to pump the well bore down to pump inlet level and then to shut the pump off for the normal off time.